1. Field of the Invention
The present invention relates to a light emitting apparatus, and more particularly, to a light emitting apparatus functioning as a display apparatus or a lighting apparatus using a light emitting element such as an organic electroluminescent element.
2. Description of the Related Art
The organic electroluminescent element is a light emitting element configured such that an organic material including a light emitting layer is disposed between two electrodes serving as a cathode and an anode respectively thereby emitting light with luminance depending on a current flowing between the electrodes. In an organic electroluminescent element for use in a lighting apparatus, each of two electrodes is formed in the shape of a solid plate, and one of them is transparent so that light can be radiated outward through the transparent electrode. Electric power to drive the organic electroluminescent element is supplied from a voltage supply generator through a power supply port and applied between the two electrodes directly or through a driving circuit.
In the case of a display apparatus of the active matrix type, organic electroluminescent elements are arranged in the form of a matrix, and the organic electroluminescent elements are independently controlled to emit light such that an image is displayed as a whole. A power supply voltage is supplied to driving circuits of respective organic electroluminescent elements via pixel power supply lines formed in the shape of a lattice or stripes extending along rows or columns of the matrix of organic electroluminescent elements. Each driving circuit is connected to a cathode or an anode of a corresponding one of the organic electroluminescent elements such that each organic electroluminescent element is driven by a supplied power supply voltage. An electrode opposite to the electrode connected to one of the driving circuits is formed in the shape of a single solid plate for common use by all organic electroluminescent elements. This common electrode is supplied with another power supply voltage, which is a ground voltage in most cases.
In display apparatuses using light emitting elements such as an organic electroluminescent element driven by an electric current, a large current flows through a power supply area extending from a power supply port to the light emitting element compared with a current in a display apparatus using a voltage-driven element such as a liquid crystal element. This current produces a voltage drop along the power supply area and thus the potential decreases with distance from the power supply port. Therefore, the power supply voltage applied to the driving circuits of the organic electroluminescent elements arranged in the form of the matrix varies depending on their location in a display area, which can produce a variation in light emission luminance among the organic electroluminescent elements. The common electrode (and also the electrode of the lighting apparatus) is formed in the shape of a single solid plate. In a case where the common electrode is located on a side through which light is radiated to the outside, the electrode is formed of a transparent electrically conductive material such as ITO. However, such an electrode using a transparent electrically-conductive material has a high sheet resistance compared with a metal electrode. Even in the plane-shape electrode, if it has a high sheet resistance, a voltage drop across the electrode can occur due to a current flowing through the electrode, which can produce a variation in luminance across the electrode.
To reduce the voltage drop along the power supply area, the power supply area may be formed using a thicker and/or wider conductive film to reduce its sheet resistance. However, the increase in the width of the power supply area results in an increase in the size of the display apparatus, which may make it difficult for the display apparatus to be used in a small-size electronic device. Another possible method to reduce the voltage drop is to provide a large number of connection terminals via which electric power is supplied to the power supply area. For example, a total of four connection terminals may be provided such that one connection terminal is located in each of four corners of the display apparatus. However, the increase in the number of connection terminals via which to provide power supply voltages from the outside of the display apparatus may result in a reduction in space to dispose other connection terminals such as those for control signal lines, data signal lines, etc. Furthermore, the provision of connection terminals at many locations may cause an increase in members or materials such as a conductive film associated with the connection terminals, which may result in an increase in cost.
U.S. Patent Application Publication No. 2006/0284803 discloses a display apparatus in which a current supply line with a large width is provided at a side of a display area in which organic electroluminescent elements are arranged in the form of a matrix such that a current is supplied into the display area across the side of the display area. The current supply line is partially cut in a direction parallel to the side of the display area such that one end of the current supply line is partially divided by slits. The partially divided end of the current supply line is connected to a power supply. A current fed from the power supply into the current supply line flows through a detour path around the slits, which leads to an improvement in uniformity of potential in an area close to a boundary to the display area compared with a power supply line having no slits.
In the above-described display apparatus disclosed in U.S. Patent Application Publication No. 2006/0284803, although the terminal for connection with the power supply is located at the end of the power supply line, it is possible to achieve a voltage drop similar to that which occurs in a power supply line having a connection terminal located in the center of a side of the power supply line. However, this configuration still has an uneliminated voltage drop along the side from its center to its end.